This invention relates to a defrosting apparatus for removing frost deposited on a cooler of a refrigerator, an air conditioner or the like.
Deposition of frost on a cooler of a refrigerator or an air conditioner results in an undesirable reduction of the heat exchange efficiency of the cooler hence, in a reduced cooling ability. Therefore, in a refrigerator, for example, a defrosting heater is provided, and, while stopping the operation of the refrigerator after the refrigerating operation of the refrigerator over a predetermined period of time, current is supplied to the defrosting heater so as to remove frost by melting by the heat produced by the defrosting heater thereby preventing the undesirable reduction of the refrigerating ability. As a prior art example of the defrosting heater commonly used in the refrigerator, there is a heater structure in which a metal wire such as a nichrome wire or a nickel-copper wire is inserted in a protective tube of material such as aluminum.
The prior art defrosting heater does not possess the function of self-temperature control and is such that it generates and maintains a constant quantity of heat regardless of the amount of frost deposited on the cooler and regardless of the distribution of frost. Thus, complete removal of frost from various portions of the evaporator is not attained at the same time. In other words, complete removal of frost from the cooler portion having a largest amount of frost deposited thereon is delayed relative to the remaining portions. In order to detect complete removal of frost from all the portions of the cooler, therefore, a temperature sensor such as a thermistor has been mounted on the cooler portion where complete removal of frost is attained latest, and the defrosting process has been regarded to be completed when the temperature sensor continuously sensing the temperature at that portion senses a predetermined temperature in the course of defrosting by the defrosting heater. Further, taking into consideration the variation in the amount of deposited frost varying depending on the season of a year and also the variation in the distribution of deposited frost varying depending on the disposition of stuffs such as foodstuffs stored in the refrigerator, the defrosting cnditions including the defrosting temperature and duration had to be suitably determined to ensure sufficient removal of frost in each of the above cases. Consequently, the temperature of a cooler portion, where the amount of deposited frost is relatively small and removal of frost is completed earlier than another portion, becomes unnecessarily high, and the current is supplied for an unnecessarily long period of time. This means that large temperature differences occur between various portions of the cooler at the time of termination of current supply to the defrosting heater. The prior art defrosting system, according to which a portion of the evaporator is heated up to an unnecessarily high temperature, has therefore been defective in that a long period of time is required for reducing the temperature of the cooler before the operation of the refrigerator is re-started after the complete removal of frost, and that a large quantity of power is inevitably consumed. The prior art defrosting system has also been defective in that an unnecessarily large quantity of power has been required for generating heat from the defrosting heater. The prior art defrosting system has further been defective in that the temperature of stuffs such as foodstuffs stored in the refrigerator tends to be raised.
As a means for obviating the prior art defects pointed out above, Japanese Patent Application Laid-open No. 101533/79 laid open to the public on Aug. 10, 1979 (Application No. 7281/78 filed on Jan. 27, 1978) discloses a defrosting control apparatus in which a positive-characteristic thermistor exhibiting a positive temperature coefficient of resistance is used in place of the prior art defrosting heater or is incorporated in a portion of the prior art defrosting heater, and the current flowing through the defrosting heater is interrupted when the heater current decreases to a predetermined constant level. However, the resistance value of the heater disposed in close proximity to the cooler changes inevitably after a long period of time of use, because the heater is repeatedly subjected to a severe cooling and heating cycle involving frost, ice, water and heating conditions. Consequently, when the resistance value of the heater increases after a long period of time of use, the heater in the disclosed defrosting control apparatus will cease to generate heat under a condition in which frost is incompletely removed. On the other hand, when the resistance value of the heater decreases after the long period of time of use, the heater will still continue to generate heat even after the frost has been completely removed. Thus, in the disclosed defrosting control apparatus too, the threshold level controlling the cut-off timing of the heater current must be so selected as to give a margin which leads to a slight extension of the defrosting duration. Therefore, the disclosed defrosting control apparatus has had defects similar to those of the firstmentioned defrosting system in which the metal wire inserted in the protective tube is used to operate as the defrosting heater.
Another problem is how to determine the starting timing of defrosting operation in order to prevent an undesirable reduction of the refrigerating ability of the cooler. It is very difficult to accurately detect the defrosting operation starting timing since the factors including the amount of deposited frost and the distribution of deposited frost, which reduce the refrigerating ability, vary depending on the season, operating condition of the cooler etc. Thus, it has been a common practice to remove frost after a predetermined period of time of operation of the refrigerator, and this period of time has been determined by considering that the refrigerating ability of the evaporator may not be lowered even in the season in which frost tends to be deposited in a large amount.
However, a defrosting control apparatus including such means has been defective in that it carries out defrosting operation at relatively short time intervals even in the season in which the amount of deposited frost is small in spite of the fact that the refrigerating ability of the evaporator is not lowered at all in such a season, resulting in a wasteful consumption of power. The prior art defrosting control apparatus has also been defective in that the temperature of the evaporator is unnecessarily raised by the power required for the frost removal, and an extended period of time is required until the temperature of the cooler is lowered prior to re-starting the refrigerating operation. Further, a large quantity of power has been inevitably consumed.